British Ambassador Matthew Rycroft yesterday said that under the UN Charter, member states have to register decisions that are at the level of a treaty.

"That puts beyond doubt that the deal that the prime minister agreed is legally binding and irreversible in international law," he said.

The agreement was key to Britain's announcement of a referendum on June 23 on whether Britain should remain in the EU or leave the 28-nation bloc. Prime Minister David Cameron said the deal with the EU would give Britain more control over its future, lessen welfare payments to migrants, and protect Britain from being absorbed into a feared European "superstate."

The referendum is expected to be hard-fought, especially because Cameron's Conservative party is divided, and the "out" campaign has the support of popular London Mayor Boris Johnson, a Conservative and possible future prime minister.

EU President Donald Tusk, who oversaw the deal that Cameron brokered with the 27 other EU leaders, insisted Wednesday that an "out" vote in the British referendum would "change Europe forever.

And it will be a change for the worse." Rycroft said there is an issue about when the agreement with the EU enters into effect, because part of it only applies if Britain votes to remain in the EU. "But officially it has entered into force now, and it has effect on June 23 if indeed the UK votes to remain in the EU," he said.